Many aspects of the immune response depend upon precise cell movements which appear to be under some form of transmembrane control exerted through cell surface receptors. Considerable evidence suggests that intracellular contractile proteins or cytoskeletal elements are the effectors of various forms of cell movement. For many of its functions, it seems likely that the cytoskeletal system interacts with the cell membrane. The major objective of this proposal is to learn how cytoskeletal actin interacts with the cell membrane. We have developed a number of immunological and biochemical probes to study actin-membrane interactions in murine lymphocytes and in a couple of additional systems, each of which has a unique advantage. The composition of the visible cross filament which connects microvillar actin filaments to the membrane will be assessed by immuno-ultrastructural localization and in vitro analysis of the binding properties of the three actin-binding proteins (60K, 95K, 110K) which we have isolated from brush border microvilli. Similar experiments will be done to study the possible role of alpha-actinin and filamin as actin-membrane linkers in mouse lymphocytes, chicken macrophages and fibroblasts. In vivo microinjection of specific antibodies directed against these proteins will probe their function in particular membrane movements. The effect of monovalent vs. divalent anti-mouse lymphocyte cell surface IgM on the detergent solubility, G vs. F-form, and ultrastructural distribution of intracellular actin will be studied in a digitonin treated lymphocyte model. The association of membrane IgM with actin and other proteins will be evaluated by polyacrylamide gel analysis of the mlgM isolated from capped and non-capped 35S-labelled murine lymphocytes. The affinity purified anti-actin will also be used to study lymphocyte cell surface actin and its relationship to mlgM and the Fc receptor.